This paper proposes a process algebra called PPPA (priced probabilistic process algebra) by extending an existing process algebra with QoS modeling capability. This paper presents its syntax and semantics to prove that it can model not only functionality, but also non-functionality, such as reliability, performance, and cost. Finally, this paper moves to model and analyze both functionality and QoS of Web services composition in a united way based on PPPA. The paper illustrates the effectiveness with an example to prove that PPPA can support formal modeling and analyzation of QoS of web services composition.